Furnace Running But Not Heating
Furnace running but blowing cold air? Learn the common causes from pilot light issues to ignitor problems. Expert troubleshooting guide.
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Safety Warning
If you smell gas, leave your home immediately and call 911 and your gas company. Do not turn on lights or use any electrical switches.
Understanding the Problem
When your furnace runs but doesn't produce heat, it's both frustrating and uncomfortable—especially during cold weather. You can hear the system running and feel air from the vents, but the air isn't warm. This problem has several possible causes, from simple thermostat settings to more serious ignition issues. Many can be diagnosed and some even fixed without professional help.
Common Symptoms
You might be experiencing this problem if you notice:
- Furnace runs but blows cold or lukewarm air
- Blower runs continuously without heating
- House won't reach set temperature
- Furnace starts then shuts off quickly
- Pilot light is out (if applicable)
Possible Causes
This problem can be caused by several factors, listed from most to least likely:
Thermostat set to "fan only"
Check thermostat settings. Set the fan to "auto" and the system to "heat" with temperature above current room temp.
Dirty air filter
Replace the filter. Allow the system to cool down before restarting.
Pilot light out (older furnaces)
Relight the pilot following manufacturer instructions. If it won't stay lit, the thermocouple may need replacement.
Ignitor failure (newer furnaces)
Professional ignitor replacement. Ignitors are relatively inexpensive parts.
Gas supply issues
Check that the gas valve to the furnace is open. Verify other gas appliances are working. Contact gas company if widespread.
Flame sensor dirty
The flame sensor can be cleaned with fine sandpaper or steel wool. It's a thin metal rod in the burner area.
Blower motor issues
Professional diagnosis of blower motor, capacitor, or control board.
Limit switch tripped
Professional service to determine why overheating occurred and address the root cause.
DIY Troubleshooting Steps
Before calling a professional, try these safe troubleshooting steps:
- 1
Check thermostat settings (heat mode, fan on auto)
- 2
Replace the air filter if dirty
- 3
Verify gas valve to furnace is open
- 4
Check circuit breaker for furnace
- 5
Relight pilot light if applicable
- 6
Look for error codes on furnace display
When to Call a Professional
Contact a licensed HVAC technician if:
- You smell gas (leave house and call gas company)
- Pilot light won't stay lit
- Ignitor isn't glowing
- Furnace lights then shuts off repeatedly
- Error codes appear on display
- Basic troubleshooting doesn't help
Prevention Tips
Help prevent this issue in the future with these maintenance tips:
- Replace filters monthly during heating season
- Schedule annual furnace maintenance
- Keep area around furnace clear
- Have flame sensor cleaned during maintenance
- Address pilot/ignitor issues promptly
Estimated Repair Cost
* Actual costs may vary based on the specific cause, parts needed, and system type. We provide upfront pricing before any work begins.
Need Help With This Issue?
Our expert technicians can diagnose and fix this problem quickly.
Call (650) 540-7205Book Service OnlineQuick Facts
- Category:heating
- Urgency:Urgent
- DIY Fixes:5 of 8 causes
- Est. Cost:$0 (simple fixes) to $150-$400 (ignitor/sensor) to $500+ (blower motor)
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